Transforming growth factor-beta (TGF-beta) is a potent inhibitor of human mammary epithelial proliferation; during mammary carcinogenesis most breast cancers become resistant to TGF-beta while retaining expression of the core members of the TGF-a signaling pathway (the type I and II TGF-beta receptors, Smads 2, 3 and 4) Thus a fundamental gap in knowledge exists in terms of the mechanism by which breast cancers become resistant to TGF-beta. The type III TGF-beta receptor (TbetaRIII) has an emerging yet poorly understood role it regulating TGF-beta signaling. Based on preliminary studies the following hypothesis is proposed: Decreased TbetaRIII expression at the protein level mediated through decreased expression of the TbetaRIII stabilizing protein, GIPC, and/or by increased degradation by the ubiquitin/proteasome pathway confers resistance to TGF-beta-mediated inhibition of proliferation and defines TbetaRIII as a tumor suppressor in breast cancer. This hypothesis will be addressed by, four Specific Aims. Specific Aim 1: The effect of GIPC expression on TbetaRIII TbetaRIII functions will be established to define the mechanism by which GIPC-mediated decreases in TbetaRIII expression confers resistance to TGF-beta-mediated inhibition of proliferation in human breast cancer cells. Specific Aim 2: The site of TbetaRIII ubiquitination and effect of the ubiquitin/proteasome pathway on TbetaRIII functions will be established to define the mechanism by which ubiquitin/proteasome-mediated decreases in TbetaRIII expression confers resistance to TGF-beta-mediated inhibition of proliferation in human breast cancer cells. Specific Aim 3: The effect of altering TbetaRIII expression (through GIPC and the ubiquitin/proteasome pathway) on the tumorigenicity of human breast cancer cells will be established to define whether TbetaRIII functions as a tumor suppressor in breast cancer cells. Specific Aim 4: The expression of TbetaRIII, GIPC, ubiquitinated TbetaRIII and the E2/E3 enzymes which ubiquitinate TbetaRIII at the protein level will be established to determine whether TbetaRIII expression is decreased by GIPC or the ubiquitin/proteasome pathway in human breast cancers. These studies will determine whether TbetaRIII functions as a tumor suppressor in breast cancer, define mechanisms for its regulated expression and aid in the design of interventions to manipulate TaRIII and the TGF-beta signaling pathway for the chemoprevention and treatment of breast cancer.